A merkin (first use, according to the OED, 1617) is a pubicwig, originally worn by prostitutes after shaving their genitalia to eliminate lice or to disguise the marks of syphilis. There are many different ways of wearing a merkin, although most involve placing the merkin on the vulva or the scrotum.
The term is also applied to decorative (typically sequinned) patches commonly sold in sets with nipple tassels or "pasties" and are enjoying new popularity as part of the costume of new burlesque adult entertainment, amongst whom the most famous modern day artistes are Dita Von Teese and Immodesty Blaize.
Houghton Mifflin's American Heritage Dictionary of the English Language, 4th edition describes the term's etymology as stemming from an "alteration of obsolete malkin, lower-class woman, mop, from Middle English; from Malkin, diminutive of the personal name Matilda."[1]
It has also been suggested that, in the period when boy actors played female parts, they would cover their genitals with a merkin so they could expose themselves as women in bawdy scenes.[2